1. Field of the Invention
The present invention relates generally to the field of electromagnetically excited heating materials and articles made therefrom, particularly those used in food packaging for improving the cooking, heating or browning of food in microwave ovens.
2. Related Art
Microwave heating materials have been in use and described in publications for over twenty years. An example of a microwave heating material is a microwave susceptor constructed of a coating on a film or substrate in which the coating absorbs microwave energy, converts it into heat and conducts the heat generated into food articles placed in close proximity thereto. Microwave susceptors are particularly useful in microwave food packaging to aid in browning or crisping those foods.
The field of microwave susceptor packaging technology includes numerous attempts to optimize heating, browning and crisping of food cooked in microwave ovens. Such attempts include German Patent Number 2,160,924 issued 1971, which first described a food wrap using a carbon or metal coated material to achieve browning and crisping in a microwave oven. Later, U.S. Pat. No. 3,783,220 describes a plate or fixture which uses either carbon fibers or semiconducting coatings such as tin oxide to produce browning and crisping temperatures in a microwave oven. Yet more recently, U.S. Pat. No. 4,267,420, issued May 1981, and U.S. Pat. No. 4,641,005, issued February 1987, describe substantially similar structures of either semiconductor coatings or thin metallic coatings on thin film substrates such as polyester.
One feature common to all of the above-mentioned conventional technology is the randomness of the electric fields which are generated by a microwave oven in the vicinity of these structures. Ultimately, corresponding, randomly directed eddy currents flowing in a resistive coating such as aluminum generate heat.
Heating in a susceptor material is achieved through so-called "I.sup.2 R" losses in the material. Power dissipation P in a resistive material having a resistance R is a function of current I, where P.apprxeq.I.sup.2 R. The power dissipation P in microwave susceptor material occurs in the form of heat. The current I is the eddy current generated by microwave energy impinging on the susceptor material, while the resistance R is the resistance of the susceptor material through which the current I flows.
A more recent article disclosed by the present inventor in U.S. Pat. No. 5,412,187, issued May 1995 employs a pattern of metal which is physically discontinuous, forming fuses. The fuse patterns create a self-limiting heat source, such that when too much energy is absorbed by a particular region, the fuse breaks between adjacent susceptor areas and the heating effect is mitigated.
However, all of the most recent susceptors discussed above, have a tendency to develop random cracks and crazing of the conductive coating. This problem is thought to be due to distortion of the substrate film resulting from excess heating. Furthermore, in conventional susceptors the eddy currents are free to flow in a completely random and non-oriented fashion, as noted earlier.
The only conventional susceptors found by this inventor to be commercially viable for use in microwave oven heating applications are those which are made with thin layers of aluminum typically exhibiting a range of 75-125 .OMEGA./.quadrature. or about 0.25 optical density. Heavier thicknesses of aluminum, for instance, have been found not to provide the browning and crisping temperatures desired while frequently causing fires due to severe arcing. Thinner layers of aluminum generally do not present a fire hazard, but produce an insufficient temperature rise for the intended purpose.
However, customers for microwave food heating products routinely request faster heat production and production of higher temperature rises. No conventional solutions have proven commercially viable or technically feasible.